Ammonoids Trapped Parasites in Pearls

Everybody knows how oysters make pearls — a bit of sand or grit slips through the protective barrier of their outer shell, irritating the mollusk’s body, and the invertebrate encircles the invader with shell material. As it turns out, ammonoids — the extinct, coil-shelled cousins of modern squid and nautilus — made pearls, too, but they did so to stave off the onslaught of prehistoric parasites.

The idea that ammonoids made pearls isn’t new. For years, paleontologists had noticed strange pits on the internal casts of Devonian-age ammonoid shells, and this indicated that the inside of the shell — which had since dissolved away — was pockmarked with bumps. What these pits represented depended upon who you asked, but in 1960 the paleontologist Michael House proposed they were the traces of pearls.

The pearl hypothesis was difficult to confirm. The indentations — now called “Housian pits” by scientists — could have represented true pearls, sites for muscle attachments, or something else. Now, in a paper just published in Acta Palaeontologica Polonica, Universität Zürich paleontologist Christian Klug and colleagues have solved the mystery.

A restoration of the ammonite Asteroceras, resembling what the ammonoids in the study might have looked like in life. From Wikipedia.

After sampling a variety of ammonoid specimens spanning a range of time between 405 and 385 million years ago, Klug and his co-authors found that many of these fossils had Housian pits. Pits were not found in all ammonoids, but in some genera — such as Anarcestes and Sellanarcestes — over 50 percent of the sampled individuals had them. Whatever was creating the pits, it appeared to be relatively selective and create regular patterns in the affected species.

As proposed in the new paper, pearls are the best fit for the evidence. Ohio University paleontologist Royal Mapes, who was one of the reviewers of the paper, agrees. “I have complete confidence that the pits represent pearls,” he said via e-mail.

House was right, but the ammonoid pearls were not fit to be strung along a necklace. The ammonoids were making blister pearls — a particular type of pearl that is created on the interior of the shell when something irritates the mollusk’s soft mantle. Given that ammonoids added to their spiral shells as they grew, moving their soft bodies into the forwardmost chambers as they did so, new shell material eventually overgrew the source of the irritation and encapsulated it.

The biological origin of the irritating agent became clear when Klug and colleagues cut into preserved shells of Sellanarcestes. The cross-sections of the blister pearls revealed tiny tubes that attached to the shell wall, denoting the presence of a living organism rather than just a piece of sand. This evidence, added to the variation in shape, size, and presence of these pits in select species over time, is more consistent with parasitism than other proposed causes, such as certain shell types being more susceptible to having sand enter the body. More than that, the changes in pit type in different ammonoid species over a 20 million year swath suggests a co-evolution between parasite and host. The parasites were changing to keep up with the rapid adaptation of the ammonoids. Reached by e-mail, Klug noted that “The ammonoids’ reactions [to the parasites] changed through ammonoid evolution,” but cautions that “we cannot study the detailed reactions” of these interactions since both the hosts and parasites are long gone.

The precise type of parasite which infested the ammonoids is unknown, but Klug and colleagues have an inkling. Trematode worms — or “flukes” — commonly afflict mollusks today, and these invaders often use mollusks as intermediate hosts in their life cycles. The fact that this evidence of mollusk parasitism coincides with an evolutionary radiation of early, jawed fish may indicate that the trematodes were using the ammonoids as a pathway to their vertebrate hosts, although this hypothesis has yet to be confirmed with hard fossil evidence. Likewise, the identification of the parasite must remain provisional. It is possible that some other creature was responsible, and without a certain identification it is difficult to know how the parasites entered the bodies of their hosts. “So far, no ammonite has been found with really well-preserved soft tissues,” Klug said, “so up to now, it is impossible to tell what these parasites did to their host.”

Nevertheless, bivalve mollusk specimens of about the same geological age as the ammonoids also have Housian pits, and the patterns are similar to those in modern-day shells which have been infested with trematodes. The flukes remain the best potential culprits. Now that paleontologists know what to look for, other incidences of parasitism are likely to show up. Citing his own ongoing research into the mystery, Mapes stated “it seems to be clear that the record of blister pearl formation and related structures in cephalopods in the Paleozoic is even more complex and diverse than that reported by Klug [and co-authors].”

There is much which remains unknown about these interactions, but, from the evidence recovered so far, paleontologists are beginning to map an evolutionary arms race which has been going on for at least 400 million years.

Image: Two specimens of the ammonoid Sellanarcestes collected from Morocco. The white arrows mark the presence of Housian pits. From De Baets et al, 2011 (with permission from the Institute of Paleobiology, publisher of Acta Palaeontologica Polonica.)

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